a) Field of the Invention
This invention relates to an ink-jet recording sheet (hereinafter simply called "a recording sheet"), and more specifically to a recording sheet having an ink receiving layer - which imparts excellent water-proofness and moisture resistance to printed characters, pictures, patterns or the like (hereinafter collectively called "printed marks" for the sake of brevity), is excellent in ink absorbency and ink-color-producing ability, provides stable printed marks of high quality, and is also outstanding in the transportability and blocking resistance inside a printer--and also to a coating formulation for producing the sheet.
b) Description of the Related Art
Ink-jet recording is to perform recording of an image, characters or the like by causing tiny droplets of an ink to fly and stick on a recording sheet made of paper or the like. Various operation principles have been proposed including, for example, the electrostatic attraction method, the method that mechanical vibrations or displacements are applied to an ink by means of a piezoelectric element, and the method that an ink is heated to bubble and the resulting pressure is used. As a recording method which permits high-speed recording, produces less noise and enables high-quality printing and multicolor printing, ink-jet recording is finding ever-increasing utility for various applications.
For use in such ink-jet recording, various recording sheets have been proposed, including recording sheets provided on paper or like bases with ink-receiving layers, which are composed primarily of various pigments and resins, or recording sheets containing porous pigments incorporated in themselves upon making paper so that prompt absorption of ink and formation of well-defined ink dots can be assured without a reduction in print quality due to blotting and/or bleeding of the ink adhered on the recording sheets.
For example, JP Kokai No. 57-82085 discloses a recording sheet which has an ink-receiving layer composed of a water-soluble resin and containing an inorganic pigment and an organic pigment, and JP Kokai No. 62-268682 discloses a recording layer which carries an ink-receiving layer composed of a silanol-containing polyvinyl alcohol copolymer and containing fine powdery silica.
However, keeping the step with improvements in the performance of ink-jet recording machines, such as high-speed recording, high-density recording and full-color recording, and the resulting expansion of their application fields, it has also become necessary for recording sheets to have high-level characteristics such as:
(1) Prompt ink absorption and large ink absorption capacity.
(2) High color-producing ability for inks.
(3) High surface strength on the ink-receiving layer.
(4) High waterproofness of the base material so that the base material will not develop roughness or curling by adhered ink.
(5) Good mark storability, such as waterproofness and ozone resistance, after printing of marks on the ink-receiving layer.
(6) No quality changes of the ink-receiving layer along the passage of time.
To meet these requirements, it has been proposed or studied to use a porous pigment or water-soluble polymer having excellent ink absorbency as a component of an ink-receiving layer to be placed on a recording sheet, to use a latex for an improvement in the water-proofness of an ink-receiving layer, and to use as a base material itself a synthetic paper sheet, plastic sheet or the like equipped with waterproofness.
However, those making use of paper as a base material or a water-soluble resin alone as an ink-receiving layer have poor waterproofness at their ink-receiving layers, leading to a drawback in that blotting takes place at parts printed with ink and marks so formed are hence inferior in definition. On the other hand, recording sheets making use of a synthetic paper sheet or plastic film as a base material and those making use of a latex as a resin for the formation of an ink-receiving layer involve problems in the adhesion between the ink-receiving layer and the base material, the ink absorbency of the ink-receiving layer and the drying property of applied ink.
To improve the waterproofness and moisture resistance of printed images of a recording sheet, it has been the general practice to arrange a protective layer over an ink-receiving layer or to add a mordant or the like in an ink-receiving layer. As a method for the arrangement of a protective layer, a hydrophobic resin may be coated or a film may be laminated over an ink-receiving layer after printing images thereon. Although such a method can bring about improvements in waterproofness and moisture resistance, it requires many steps and therefore is not preferred for the formation of images from the standpoint of price.
Concerning the method which features the addition of a mordant or the like in an ink-receiving layer, dyes employed in ink-jet color inks are direct dyes or acid dyes, the molecule of each of which contains an anionic carboxyl or sulfonic group. To improve the waterproofness and fixability of images formed with these dyes, a cationic mordant or the like is added to an ink-receiving layer. The bonding between the mordant and its associated dye is however ionic bonding, which is prone to dissociation in the present of water. A limitation is therefore imposed on the waterproofness and moisture resistance of images so formed.